1. Field of the Invention
The present invention relates to reflective optical sensors and image forming apparatuses and, more particularly, to a reflective optical sensor that detects at least one of a position or a toner density of a toner pattern and an image forming apparatus including the same.
2. Description of the Related Art
As an image forming apparatus forming an image with toner, a copier, a printer, a plotter, a facsimile device, a multi-function printer (MFP), and the like are widely known. With such image forming apparatuses, in general, an electrostatic latent image is formed on a surface of a drum having photosensitivity (hereinafter, also referred to as “photosensitive drum” for convenience sake) and so-called development is performed in which the toner is adhered onto the electrostatic latent image to obtain a “toner image”.
To obtain a good toner image, the amount of toner provided for the development of electrostatic latent images needs to be appropriate. There are various known development methods such as a method using “a two-component developer containing toner and carrier” and a method using a mono-component developer containing toner only. The amount of toner provided to a developing unit where an electrostatic latent image is developed is also referred to as “toner density”.
When the toner density is too low, a sufficient amount of toner is not provided to the electrostatic latent image resulting in the image output from the image forming apparatus (output image) having an insufficient density. On the other hand, when the toner density is too high, the density distribution of the output image is concentrated on a “high density side”, making the image hard to see. To obtain a good output image, the toner density needs to be within an appropriate range.
Consequently, techniques are widely employed to control the toner density within the appropriate range. According to the techniques: a pattern is formed for detecting the toner density; the pattern is irradiated with light (detecting light); and the toner density is detected based on the change in the amount of reflected light (for example, refer to Japanese Patent Application Laid-open No. H1-35466, Japanese Patent Application Laid-open No. 2004-21164, Japanese Patent Application Laid-open No. 2002-72612, Japanese Patent No. 415-4272, and Japanese Patent No. 4110027).
A conventional sensor for detecting the toner density is structured with one or two pieces of light-emitting units or three pieces of light-emitting units having different wavelength characteristics, and one or two pieces of light-receiving units that receive reflected light. Furthermore, the length of the toner pattern in a main direction is set to 15 to 25 millimeters such that the light spot of detecting light as a whole illuminates the toner pattern even when the position of the toner pattern is displaced with respect to the sensor.
In the image forming apparatus, the advancement in color imaging and high speed processing is making a tandem type image forming apparatus having multiple photosensitive drums (generally, four pieces) more popular.
In the tandem type image forming apparatus, an out-of-color registration occurs in the output image if the positional relationship of toner images of the respective photosensitive drums is not appropriate. Accordingly, to control the positional relationship of toner images to make it appropriate, a method is widely used, according to which: a position detecting pattern is formed; the pattern is irradiated with light (detecting light); and the position of the pattern is detected from the temporal change in the amount of reflected light (for example, refer to Japanese Patent Application Laid-open No. 2008-276010 and Japanese Patent Application Laid-open No. 2005-238584).
While the toner density detecting process and the pattern position detecting process are in progress, a normal image forming cannot be carried out. When the toner density detecting process and the pattern position detecting process using a conventional reflective optical sensor are performed, the work efficiency of the normal image forming is deteriorated because it takes time to form the respective detecting patterns.
In addition, there has been a problem that the replacement cycle of toner cartridge is shortened because the toner used for detecting patterns is so-called “non-contributing toner” that does not contribute to normal image forming.